Although the spring migration in Maine actually is underway by early February, spring migration continues until the end of May through the Pine Tree State. Warblers, thrushes, cuckoos, flycatchers and other migratory species stream through Maine. Some breed here and others will continue north.
Each species has its own migratory schedule, timed to ensure arrival when its favored food is available. Warblers, vireos, tanagers and cuckoos glean caterpillars from the leaves of deciduous trees. Arrival before leaf-out would be a recipe for starvation. Similarly, flycatchers, swifts and night-hawks have to delay their arrival until flying insects are on the wing to provide their meals. It is no wonder that most of our migratory birds arrive in May. Species that can arrive earlier have broad diets and can subsist on seeds or residual berries. Such species include Red-winged Blackbirds, Common Grackles and Song Sparrows.
You can see the spring arrival schedule of Maine migratory breeding birds at: https://hobbes.colby.edu/arrival/
How does a bird know when to migrate? Each individual has an internal clock that is responsive to changes in the lengths of the day. When the days on the wintering grounds get longer than a critical time (or shorter than a critical time if the birds are wintering south of the equator), a bird’s internal clock induces the bird to start getting ready for the northward migration.
The internal clock induces a behavior called migratory restlessness, easily observed in migratory birds maintained in captivity. The bird is getting antsy to leave. A bird will also begin to feed voraciously to put on fat for the first leg of its migratory journey.
The weather can strongly affect bird migrations. Long-distance migrations are arduous enough without flying into a headwind. In the spring, good migratory flights are induced by periods of strong southerly winds to provide some tailwind for the migrants.
You can get a good idea of how many migrants you can expect to see in a particular morning by taking a look at a weather map.
High pressure systems (anticyclones in meteorology-speak) have a clockwise rotation so southerly winds are found on the trailing edge of a high as it moves across our continent. Low pressure systems or cyclones rotate in a counter-clockwise manner so southerly winds are found on the leading edge.
The perfect time for migration is when a low pressure system is pushing against a high pressure system to the east. Both systems produce southerly winds at their intersection and, if you are lucky, the front where the two air systems meet will produce rain. Birds will migrate at night to take advantage of the southerly winds but will be forced to land by the rain. Voila – a fallout! Birds can seem to be dripping from the trees at dawn under these conditions.
Hawk watchers are well aware of this phenomenon as well. As usual, spring hawk counts are conducted daily from Bradbury Mountain in Pownal. The best counts occur on days with southerly winds. When the wind is from the north, forget it.
NEXRAD weather radars can be used to monitor migrations. Migrating birds show up as blips on the radar screen. These blips were originally called angels before radar operators realized they were birds. A good website to delve into NEXRAD is: http://www.aos.wisc.edu/weather/wx_obs/Nexrad.html
eBird has developed a fantastic resource called Birdcast that incorporates eBird observations, NEXRAD images and weather forecasts to predict the magnitude of migration all across the United States and beyond. You should visit: http://birdcast.info/
I recommend starting with the link called A Primer for New Migration Forecast Tools. The maps are the most useful and fascinating aspect of this website. The prediction of migration intensity is made for every area of the country three hours after sundown and is updated every six hours.
Spotlight links and special interest links will keep you occupied for hours. What a resource!